Assessment of the Impact of the Myxosporean Parasite Ceratomyxa shasta on Survival of Seaward Migrating Juvenile Chinook Salmon, Oncorhynchus tshawytscha, from the Fraser River, British Columbia

Abstract

Approximately 3.3% of more than 3500 seaward migrating juvenile chinook salmon (Oncorhynchus tshawytscha) collected from the lower reaches and off the mouth of the Fraser River, British Columbia, between March and August 1985–87 were infected with Ceratomyxa shasta (Protozoa: Myxosporea). The fish were held live for up to 151 d before examination to allow the infections to become patent. The first infected fish were detected in samples taken in late May to early June, approximately 4 wk after the river water temperature had reached 10 °C. By this time, 40–65% of the fish had been collected, indicating that the majority of the juvenile chinook salmon had left the Fraser River before the infective stage of the parasite was present. Significant differences in prevalence of C. shasta were associated with both the migration route chosen by the fish and their age. Fish that used the lower flow rate North Arm had a greater prevalence (6.8%) of infection than those that migrated down the Main Arm (2.1%). Age 0 fish had a significantly higher prevalence (5.2%) of C. shasta than the age 1 group (1.5%). It is concluded that C. shasta is not a major cause of mortality of downstream migrating juvenile Fraser River chinook salmon.